(a) Field of the Invention
The present invention relates to a microwave band-pass filter whose skirting characteristic has been improved to become sharp at high and low bands.
(b) Prior Art
Needs for radio communication have become larger and larger these days and the number of radio waves used is increasing in a same area. In order to eliminate any jamming entering from other systems, therefore, it is asked that the band-pass filter (which will be hereinafter referred to as BPF) arranged in the input/output section of the radio has a sharp skirting characteristic at high and low bands. In the case of moving and portable radios, the BPF is desired to become small-sized and light-weighted.
Dielectric filters such as the coaxial and comb line type BPFs have been therefore used as the BPF arranged in the input/output section of the radio.
The BPF of the coaxial type will be briefly described with reference to FIGS. 4 and 5, of which FIG. 4 is a perspective view showing the coaxial type BPF and FIG. 5 a circuit diagram showing the coaxial type BPF in FIG. 4.
In FIG. 4, the BPF 50 of the coaxial type comprises cylindrical dielectric coaxial type resonators 51-55 each having a hole therein, and inner conductors 56-60 formed on the inner walls of the holes and connected one another in series between terminals 61 and 62 through capacitors 63-68.
The circuit of this coaxial type BPF 50 is of the capacity coupling type in which the coaxial type resonators 51-55 are connected one another in series through the capacitors 64-67, as shown in FIG. 5. Numerals 69-73 represent fringing capacities and 74-78 their equivalent coaxial lines.
The BPF of the comb line type will be now briefly described referring to FIGS. 6 and 7, of which FIG. 6 is a perspective view showing the comb line type BPF and FIG. 7 a circuit diagram showing the comb line BPF in FIG. 6.
In FIG. 6, the BPF 80 of the comb line type comprises providing holes in a rectangular-parallelepiped-shaped dielectric 81 along a center line thereof with an equal interval interposed between them, plating the inner walls of the holes to form inner conductors 82-86, forming conductors on the faces of the dielectric 81 except the top face thereof at which the holes are opened, and connecting the inner conductors 82 and 86 located on both end to terminals 87 and 88, respectively, through capacitors 89 and 90.
The equivalent circuit of this comb line type BPF 80 is of the inductive coupling type in which the resonators are induction-coupled by equivalent coaxial lines 91-94. Numerals 95-99 represent fringing capacities and 100,104 equivalent coaxial lines.
The conventional microwave BPF comprises combining the coaxial or comb line type BPFs 50 or 80 in an appropriate number of stages.
The coaxial type BPF 50 has a skirting characteristic sharp at low band but gentle at high band and its attenuation amount is not sufficient, as shown by A in FIG. 3. On the other hand, the comb line type BPF 80 has a skirting characteristic sharp at high band but gentle at low band and its attenuation amount is not sufficient, as shown by B in FIG. 3. In order to make their skirting characteristics sharp at both high and low bands, therefore, the number of the resonator stages must be increased. However, this causes the dielectric constant of the dielectric, which forms the resonators, and their dimensional irregularities to have larger influences, thereby increasing loss in pass band and worsening return loss characteristic. When the number of the resonator stages is increased like this, it also takes a longer time to adjust the resonators, thereby becoming unsuitable for mass production.
In the case of both of the coaxial and comb line type BPFs 50 and 80, a pass band where the attenuation amount is small is present, as shown by D in FIG. 3, at those frequencies which are three times the frequencies in the pass band shown left side in FIG. 3, thereby causing spurious characteristic. A low pass filter must be added to improve this spurious characteristic.